Conventional hybrid circuits usually have about 10% to 20% of their surface covered with integrated circuits. These circuits are electrically interconnected by wiring in the remaining portion of the surface and in a number of layers beneath it. The wiring width is typically 5-10 mils. By reducing this to 0.5-1 mil in width as in the multichip module technology (MCM) the area required for the interconnections, using the same number of layers, could be reduced by a factor of 100 to 1. However, since a portion of surface area is occupied by the integrated circuits and is not reduced with the wiring, there is a limit to how much the area can be reduced. While the area reduction therefore has a limit, further efficiency can be effected by eliminating one or more of the layers. Now integrated circuit chips can be packed as closely as permitted by assembly technique and repair access. In some cases the chips can be extremely close, leaving only enough separation to allow a faulty chip to be removed and replaced. With such high density packing of the integrated circuits there is increased need for heat dissipation through the ceramic substrate support to a suitable heatsink. The heat problem is compounded when there are circuit components on both sides of the ceramic support, for example, where passive components are on the opposite side from associated silicon based circuitry. A number of heat dissipation approaches are available but each has shortcomings. A high thermal conductivity material for the ceramic support, such as beryllia or aluminum nitride permits better lateral thermal conduction. But this limits the size of the overall circuit because the temperature rise at the center of the circuit relative to the edges is dependent upon the size and power dissipation per unit area. Another approach is to use high thermal conductivity blocks attached between a heat sink and the ceramic substrate support opposite the location of high power components. But it takes up considerable space that would be better used for components. A third approach is to attach the heat sink to the integrated circuit chip side, in a "chips-first" multichip module integrated circuit, but this requires that the heat be conducted through organic insulators which are not good thermal conductors. Also, immersion cooling can be used with all of its attendant problems.
A second problem which occurs as a result of the high packing density of the integrated circuit chips is that the location of the vias is restricted to being in the small spaces between the chips. This further complicates the process when the components on the opposite side cannot be aligned with the appropriate vias.